1. Field of the Invention
The present invention relates to methods and apparatuses for fabricating quantum dot functional structures, quantum dot functional structures, and optically functioning devices. More particularly, the present invention relates to a method and an apparatus for fabricating a quantum dot functional structure, a quantum dot functional structure, and an optically functioning device, which provide the following outstanding features. The features make it possible to control the diameter of and alleviate the contamination of ultra-fine particles that are expected to provide various functions resulting from the quantum size effects. The features also make it possible to provide an improved efficiency for the optically functioning device fabricated using a quantum dot functional structure, in the transparent medium of which the ultra-fine particles are distributed homogeneously.
2. Description of the Prior Art
To employ semiconductor ultra-fine particles formed of Si families of IV materials for use in an optically functioning device that can emit light in the visible spectrum, it is indispensable to provide spherical ultra-fine particles which are controlled on the order of one nanometer in diameter. Moreover, the laser ablation method is preferably applied to the fabrication of the ultra-fine particles on the order of one nanometer in diameter.
For example, FIG. 1 is a conceptual view depicting an apparatus, disclosed in Japanese Patent Disclosure No. 9-275075, for applying the laser ablation method to a conventional target material to fabricate ultra-fine particles by deposition.
Referring to FIG. 1, a laser light beam is emitted from an excimer laser source 1 and travels through an optical system constituted by a slit 2, a condenser lens 3, a mirror 4, and a laser light inlet window 5 to be guided into a vacuum reaction chamber 6, where the laser light beam is focused on and thus radiates the surface of a target material 8 placed in a target holder 7, which is arranged inside the vacuum reaction chamber 6.
In addition, there is arranged a deposition substrate 9 in a direction normal to the surface of the target material 8. Substances detached or ejected from the target material 8 by laser ablation are captured or deposited on the deposition substrate 9.
An explanation will be given below in more detail to a case where semiconductor ultra-fine particles are fabricated with Si being employed as the target material in the apparatus configured as described above.
First, the vacuum reaction chamber 6 is pumped down to an ultra-high vacuum of pressure 1×10−8 Torr by means of an ultra-high vacuum exhaust system 10, which is mainly constituted by a turbo-molecular pump, and then the ultra-high vacuum exhaust system 10 is closed.
Subsequently, a helium (He) gas is introduced through a rare-gas guide line 11 into the vacuum reaction chamber 6. The vacuum reaction chamber 6 is held at a constant pressure (of 1.0 to 20.0 Torr) with the low-pressure rare gas (He), the flow of which is controlled by means of a mass-flow controller 12 and which is differentially exhausted by means of a differential exhaust system 13 mainly consisting of a dry rotary pump. In the He gas atmosphere kept at a pressure of a few Torr, the surface of the target material is radiated with a laser light beam of a high-energy density (e.g., 1.0 J/cm2 or greater) to cause the substances to be detached or ejected from the target material.
The detached substance gives kinetic energy to the surrounding gas molecules, which are in turn urged to condense and grow in the gas atmosphere into ultra-fine particles of a few to a few tens of nanometers in diameter, the ultra-fine particles being deposited on the deposition substrate 9.
Originally, since the IV-group semiconductors are an indirect bandgap material, their bandgap transitions cannot be dispensed with phonons. The materials naturally cause much heat to be generated in their recombination, thus providing significantly decreased radiative recombination probability. However, the material shaped in ultra-fine particles having a diameter of a few nanometers causes the wave number selection rule to be relaxed in bandgap transitions and the oscillator strength to be increased. This in turn increases the probability of occurrence of radiative electron-hole pair recombination, thereby making it possible to provide intense light emission.
Here, the wavelength of emitted light (i.e., the energy of emitted photons) is controlled by making use of an increase in absorption edge emission energy (corresponding to bandgap Eg) provided by the quantum confinement effect resulted from a decrease in diameter of ultra-fine particles. FIG. 2 is an explanatory graph showing the correlation between the diameter of the aforementioned ultra-fine particles and the absorption edge emission energy thereof.
That is, to emit light at a single wavelength, it is indispensable to make the diameter of the ultra-fine particles uniform. If ultra-fine particles of a diameter corresponding to the emission wavelength can be generated and deposited within as narrow a diameter distribution as possible, it is made possible to fabricate an optically functioning device for emitting light of a single color.
As described in the aforementioned prior art, it is required to generate and deposit ultra-fine particles having a particle diameter distribution controlled to provide a single diameter of a few nanometers in order to fabricate an optically functioning device for emitting light at a single wavelength using semiconductor ultra-fine particles.
The prior art makes it possible to control the mean particle diameter by selecting as appropriate the pressure of an atmospheric rare gas or the distance between the target material and the deposition substrate. However, the prior art provides a still broad particle diameter distribution. Thus, it is difficult to obtain semiconductor ultra-fine particles of a uniform diameter distribution having, for example, a geometric standard deviation σg of 1.2 or less.
That is, this means that more aggressive particle diameter control is required. In addition, nm-sized ultra-fine particles are very sensitive to the contamination of impurities or defects due to their high surface atom ratio (e.g., about 40% at a particle diameter of 5 nm).
That is, it is required to provide a clean and damage-less process as a method for generating and depositing the particles. Moreover, adhering and depositing semiconductor ultra-fine particles directly onto a deposition substrate as in the prior art would tend to result in a thin film of a porous structure formed of a deposit of ultra-fine particles.
Suppose that electrodes are connected to such a porous structure to allow it to function as an optically functioning device. In this case, it may be required to optimize the structure somehow. On the other hand, in order to derive the quantum size effect originally provided for spherical ultra-fine particles to implement a new optical function representative of light emission, further optimized shape and structure may be required such as a structure having particles distributed homogeneously in a stable transparent medium.
In addition, since nm-sized ultra-fine particles have a very sensitive surface as described above, it may become necessary to form a quantum dot functional structure having the particles being homogeneously distributed in a stable transparent medium.
In addition, in order to obtain fine particles having a specified particle diameter, a fine particle classifier may be used for classifying the diameter of fine particles using the mobility which is dependent on the particle diameter. Such a fine particle classifier has been used for performance test of high-performance air filters for collecting and separating sub-micron fine particles with high efficiency, and for generating standard fine particles and measuring the particle diameter upon monitoring of cleaned atmosphere. The mobility employed for classifying the diameter of particles includes mainly the electrical mobility of charged particles in an electro-static field and the dynamic mobility caused by gravitational force. In addition, the aforementioned fine particle classifier has two main structures: a double cylinder and a disk type structure.
FIG. 3 is a schematic view illustrating the structure of a prior-art differential electrical mobility classifier, introduced in Japanese Journal of Aerosol Research Vol. 2, No. 2, p 106 (1987) or in Japanese Journal of Powder Engineering Society Vol. 21, No. 12, p753 (1984). This differential electrical mobility classifier has a double cylindrical structure comprising an outer cylinder (of radius R1) 19 and an inner cylinder (of radius R2) 20 disposed inside the outer cylinder 19 concentrically with the outer cylinder 19. Referring to FIG. 3, charged fine particles 21 are transported by a carrier gas 22 to flow into the double cylinder classifier from the upper end portion thereof to be mixed with clean air or a sheath gas 23 flowing therein. The mixture gas of the charged fine particles 21 and the sheath gas 23 flows as a laminar flow over a length of L through the double cylinder portion. An electrostatic field is applied to this double cylinder portion with a direct current power supply 24 in a direction perpendicular to the flow of said mixture gas. This causes the charged fine particles 21 to draw an orbit in accordance with the electrical mobility of each particle. Since said electrical mobility is dependent on the diameter of fine particles, only those fine particles having a specific diameter arrive at a lower slit 25 and then are classified to be taken out of a carrier gas exhaust vent 26. The fine particles of other diameters are exhausted from a sheath gas exhaust vent 27 in conjunction with the sheath gas 23 or caused to move to and adhere to an inner collector electrode 28.
On the other hand, as a prior art fine particle classifier, a dynamic mobility classifier having a disk structure is disclosed in Japanese Patent Disclosure No. 9-269288. FIG. 4 is a schematic view illustrating the structure of the dynamic mobility classifier of the aforementioned disk type.
This disk-type dynamic mobility classifier comprises a disk-shaped upper disk 31, a disk-shaped lower disk 32 disposed opposite to and spaced apart by a predetermined distance from the upper disk 31, and a particle collector portion 33 attached to the lower disk 32 and disposed opposite to the upper disk 31. There is formed a cylindrical central suction duct 34, having an opening at one end thereof, on the central portion of the upper disk 31. There are formed a plurality of holes or slits 35 for introducing a carrier gas in the vicinity of the rim portion of the disk in the outward radial direction from the central suction duct 34. The lower disk 32 is provided with substantially the same diameter as that of the upper disk 31 and disposed generally in concentric relationship therewith. There are formed a plurality of holes or slits 36 for emitting a carrier gas on a portion apart by a predetermined distance in the outward radial direction from the center of the lower disk 32. The slits 35 provided on the upper disk 31 and the slits 36 provided on the lower disk 32 are a plurality of slits formed in an annular shape along a predetermined circumference, spaced apart at certain intervals. The distance radially outward from the center of the disk to the position of the slits 36 provided on the lower disk 32 is designed to be less than the distance radially outward from the center of the disk to the slits 35 provided on the upper disk 31. Between the upper disk 31 and the lower disk 32, there is defined a space or a classifying region 37. On the central portion of the particle collector portion 33, there is formed a cylindrical withdrawal duct 38 having an opening at one end thereof. The particle collector portion 33 is adapted to discharge classified fine particles from the withdrawal duct 38 in conjunction with the carrier gas.
Referring to FIG. 4, the classifying region 37 is formed in a space defined between the upper disk 31 and the lower disk 32, arranged to be concentric and parallel to each other. A sheath gas or an air flow 39 is introduced into the classifying region 37 from the periphery of the upper and lower disks 31, 32, being supplied from the outer rim inwardly in the radial direction. The air flow 39 takes place as a centripetal laminar flow through the classifying region 37 and is exhausted from the central suction duct 34 (indicated by arrow A1 in FIG. 4). Fine particles 40 are transported with a carrier gas 41 to be guided from the slits 35 provided on the upper disk 31 into the classifying region 37. The fine particles 40, which have been guided from the slits 35 provided on the upper disk 31 into the classifying region 37, are moved with the air flow 39 toward the center axis as well as drop from the upper disk 31 toward the lower disk 32 due to the gravitational field. Since the drop speed is dependent on the diameter of the fine particles 40, only those fine particles having a certain diameter are allowed to reach the slits 36 arranged on the lower disk 32, thus being classified and taken out of the withdrawal duct 38 (indicated by arrow A2 in FIG. 4). The particles having other diameters are exhausted from the central suction duct 34 in conjunction with the air flow or moved to the lower disk 32 to adhere to the surface thereof.
In the field of such fine particle classification technology, known is that the physical properties of ultra-fine particles having diameters from a few to a few tens of nanometers vary depending on the particle diameter. For example, the energy gap of semiconductor ultra-fine particles increases as the particle diameter decreases. Attempts have been made to create new devices by making use of the physical properties of the aforementioned semiconductor ultra-fine particles. As a substance for forming the aforementioned new device, Si has received attention. In this context, attempts have been made to create ultra-fine particles of Si having diameters from a few to a few tens of nanometers by making use of the pulse laser ablation in a rare gas. To create a new device employing the Si ultra-fine particles, it is necessary to classify the Si ultra-fine particles having various diameters on the order of a few to a few tens of nanometers and thus extract those Si ultra-fine particles having a narrow particle diameter distribution enough to regard the particles as having a single diameter. In addition, the mean particle diameter of the Si ultra-fine particles to be classified can be preferably varied.
On the other hand, the prior art disk type dynamic mobility classifier shown in FIG. 4 is adapted to classify fine particles having generally sub-micron diameters, employing the gravitational field for classifying the particle diameter. Since the gravitational field is constant, it is necessary to vary the flow rate of the air flow 39 in order to vary the mean particle diameter of the ultra-fine particles being classified. A variation in mean particle diameter of nm-sized ultra-fine particles requires a fine variation in flow rate of the aforementioned air flow 39. It is extremely difficult to control this fine variation in flow rate and to stabilize the flow rate.
Furthermore, in order to classify ultra-fine particles having sub-micron or less diameters without increasing the size of the aforementioned disk type dynamic mobility classifier (i.e., without increasing the projective distance of the annually formed slits 35, 36), it is necessary to apply a force greater in magnitude than the gravitational force to the ultra-fine particles in a direction perpendicular to a sheath gas flow or the air flow 39 (in the direction from the upper disk 31 to the lower disk 32) in the classifying region 37.
In addition, as a method for improving the classification resolution of the ultra-fine particles, such a technique is available that increases the classifying region from one stage to multiple stages to increase the number of times of classification. Referring to the double cylinder classifier shown in FIG. 3, for example, the double cylinder classifier described in the Japanese Journal of Powder Engineering Society Vol.21, No.12, p753 (1984) has the dimensions of the classifying region of L=400 mm, R2=15 mm, and R1=25 mm. In this context, suppose a cylindrical classifying region is further added to the outer periphery of the aforementioned double cylinder classifier to provide multiple stages of the classifying region. In this case, the overall dimensions of the classifier would be significantly increased. Therefore, it is necessary to employ a structure other than that of the double cylinder type to make the overall dimensions of the classifier small.